Neil Gibson, the Gold King and former senator from "some Western state", approaches Sherlock Holmes to investigate the murder of his wife Maria in order to clear his children's governess, Grace Dunbar, of the crime. It soon emerges that Mr Gibson's marriage had been unhappy and he treated his wife very badly. He had fallen in love with her when he met her in Brazil, but soon realised they had nothing in common. He became attracted to Miss Dunbar; since he could not marry her, he had attempted to please her in other ways, such as trying to help people less fortunate than himself.

Maria Gibson was found lying in a pool of blood on Thor Bridge with a bullet through the head and note from the governess, agreeing to a meeting at that location, in her hand. A recently discharged revolver with one shot fired is found in Miss Dunbar's wardrobe. Holmes agrees to look at the situation in spite of the damning evidence.

From the outset, Holmes observes some rather odd things about the case. How could Miss Dunbar so coolly and rationally have planned and carried out the murder and then carelessly tossed the murder weapon into her wardrobe? What was the strange chip on the underside of the bridge's stone balustrade? Why was Mrs Gibson clutching the note from Miss Dunbar when she died? If the murder weapon was one of a matched pair of pistols, why couldn't the other one be found in Mr Gibson's collection?